The present invention relates to a cordless telephone set, and more particularly to a cordless telephone system in which a master unit is provided with various control functions which are controllable by at least one of a plurality of sub-units, any sub-unit can be called from an external commercial telephone line, transfer between sub-units is permitted and speech between sub-units is permitted through the master unit.
It is an object of the present invention to provide a new and improved cordless telephone set and application apparatus thereof based on the following problems (1)-(5) encountered in the conventional cordless telephone set.
(1) In a conventional cordless telephone set, one sub-unit is used for one master unit. Accordingly, if a person who carries the sub-unit is present at a pool side, bath room or rest room, it is troublesome to tell him/her that he/she is having a telephone call, and in some case it is difficult to tell him/her. In one aspect of the present invention, a plurality of sub-units are associated with the master unit and means for transferring the telephone call to another sub-unit is provided. When there is no speech with an external telephone, communication and speech between sub-units are permitted to efficiently use the telephone set.
In the case of using one master unit in common for a plurality of sub-units in a conventional set, it is impossible for the sub-units to have mutual communication between any two sub-units since they are designed to use a transmitted signal and a received signal of the same frequency.
The present invention makes possible a mutual communication between any two sub-units by way of a master unit. When any one sub-unit A transmits a speech signal, which is received by and transmitted from the master unit, another sub-unit B can receive a transmitted speech signal from the master unit. In such a communication, if the sub-unit B transmits a signal wave, it will interfere with the signal wave transmitted from the sub-unit A to disturb and prevent the signal reception of the master unit. In order to avoid this drawback, the sub-unit B is inhibited from transmitting its own speech signal when receiving from another sub-unit through the master unit. Thus any two sub-units A and B can conveniently communicate with each other using a press-talk system.
(2) In non-speech mode, the master unit and dialing signal senders of the sub-units are idle. Accordingly, by providing various control functions in the master unit and controlling them by various signals generated by complex combinations of dial digits of the sub-unit, the telephone set may be rendered to function as an automatic responding and recording telephone set, the sub-unit may be permitted to reproduce the speech done during absence, a light may be turned on and off or a garage door may be opened and closed so that the cordless telephone set is more effectively used. Since all of the functions of the master unit can be controlled by the sub-units, the master unit may be placed at a high position not accessible by human being so that a range of reach of its electromagnetic wave is expanded. If it is housed in a locker, a mounting space can be saved and trouble by a cord is also eliminated.
(3) A conventional cordless telephone system has also been arranged such that the master-unit is internally provided with an intercom device and the cordless telephone could be manually switched to operate as an intercom device. According to one aspect of the present invention, the master unit can be controlled by a signal from any one sub-unit to relay the sub-unit and an intercom device away from the master unit. Thus, the sub-unit and the intercom device can communicate with each other through the master unit. Also, in the prior art intercom set, when an attendant hears a calling tone, he/she goes to a place of the master unit and can communicate by a press-to-talk method. Accordingly, if he/she cannot go to the place of master unit because, for example, he/she is taking a bath or in a rest room, he/she cannot respond to the call. If he/she is at a distant place that he/she cannot hear the calling tone, he/she cannot therefore respond. Such trouble also occurs in a commercial telephone set and it is the cordless telephone set that has been invented to solve this problem. Namely, if he/she always carries a sub-unit, he/she can hear the calling tone wherever he/she is and he/she can respond immediately. If the external telephone set can be replaced by the intercom set, the disadvantage encountered in the conventional intercom set can be eliminated as is done for the disadvantage of the cordless telephone set. (4) In a cordless telephone set having one master unit and a plurality of sub-units, if the sub-units and master unit have press-to-talk functions, communication between sub-units is permitted through the master unit, as explained above. However, this system has the following problem. It is necessary that a transceiver and the master unit are continuously in operation during the speech between sub-units When the speech terminates, the initial state must be immediately restored in order to prevent trouble in the speech with an ordinary external telephone line. When an external line is to be transferred from a sub-unit A to a sub-unit B, the external line must be kept on hold while the speech and exchange are done between the sub-units. Further, speech between the sub-unit and the external line must be permitted immediately after the termination of communication between the sub-units. In order to immediately reset the master unit at the end of speech between the sub-units, the simplest approach is to use an electromagnetic wave emitted from the sub-unit. However, if the transmitted wave between the sub-units ceases for a long period, a timer circuit is reset and the hold of the external line is released and the speech is interrupted. If a time constant of the timer circuit is set too long, it operates for a long time after the end of speech and this may cause various problems. As shown herein later in FIGS. 6 and 7, in a system which issues a stop signal then, an additional encoder and decoder are needed and price is higher accordingly. Further, the sub-unit is of large size.
In other aspect of the present invention, when the sub-unit B is switched from the press-to-talk mode to a normal speech mode, a predetermined signal is automatically sent out for a very short period so that the timer circuit of the master unit is reset by that signal. A circuit for sending a signal at the start of speech may be used as it is as a circuit for transmitting and receiving the predetermined signal at the end of speech in order to prevent increases of the system cost and size.
(5) In the conventional cordless telephone set, there are two operation modes, stand-by and speech. In the stand-by mode, only a calling tone is reproduced by a speaker and no speech tone is reproduced. In the speech mode, since an electromagnetic wave is emitted, the operation of another unit can be checked and not be used. Accordingly, there is no risk that the speech tone is intercepted. However, in the system in which a plurality of sub-units are associated with the master unit, speech between the sub-units is permitted through the master unit in the press-to-talk system. Accordingly, if the switch is thrown to the press-to-talk mode, the transmission tone between other units can be readily intercepted and such an interception may not be detected by other unit because no electromagnetic wave is emitted in such a mode.
In other aspect of the present invention, the above disadvantage is eliminated by rendering receivers of other sub-units nonoperative while one sub-unit is in speech.